Chap4 Logctrl Ops

8/7/2019 Chap4 Logctrl Ops

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CArithmetic operators

bitwise complement (~i)~

Right shift (i > 1)>>,


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Bit-wise Logical operations

Bitwise AND operation(w AND f) andwf floc, d d (floc) & (w) j = j & i;

(w AND literal) andlw k w (0xkk) & (w) j = j & 0xkk;

Bitwise OR operation

(w OR f) iorwf floc, d d (floc) | (w) j = j | i;

(w OR literal) iorlw floc, d d 0xkk| (w) j = j | 0xkk;

Bitwise XOR operation

(w XOR f) xorwf floc, d d (floc) ^ (w) j = j ^ i;

(w XOR literal) xorlw floc, d d 0xkk^ (w) j = j ^ 0xkk

Bitwise complement operation;

(~ f) comf floc, d d ~ (floc) j = ~ i ;


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Clearing a group of

bits

Data Memory

Location contents

(i) 0x020 0x2C

(j) 0x021Clear upper four bits of i .

In C:

i = i & 0x0f;

In PIC18 assembly

movf 0x020, w ; w = i

andlw 0x0f ; w = w & 0x0f

movwf 0x020 ; i = w

i = 0x2C = 0010 1100

&&&& &&&&mask= 0x0F = 0000 1111

---------result = 0000 1100

= 0x0C

AND: mask bit = 1, result bit is same as operand.

mask bit = 0, result bit is cleared

The mask

0xB2

(k) 0x022 0x8A


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Setting a group of

bits

Data Memory

Location contents

(i) 0x020 0x2C

(j) 0x021Set bits b3:b1 of j

In C:

j = j | 0x0E;

In PIC18 assemblymovf 0x021, w ; w = j

iorlw 0x0E ; w = w | 0x0E

movwf 0x021 ; j = w

j = 0xB2 = 1011 0010

|||| ||||mask= 0x0E = 0000 1110

---------result = 1011 1110

= 0xBE

OR: mask bit = 0, result bit is same as operand.

mask bit = 1, result bit is set

The mask

0xB2

(k) 0x022 0x8A


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Complementing a

group of bits

Data Memory

Location contents

(i) 0x020 0x2C

(j) 0x021Complement bits b7:b6 of k

In C:

k = k ^ 0xC0;

In PIC18 assemblymovf 0x022, w ; w = k

xorlw 0xC0 ; w = w ^ 0xC0

movwf 0x022 ; k = w

k = 0x8A = 1000 1010

^^^^ ^^^^mask= 0xC0 = 1100 0000

---------result = 0100 1010

= 0x4A

XOR: mask bit = 0, result bit is same as operand.

mask bit = 1, result bit is complemented

The mask

0xB2

(k) 0x022 0x8A


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Complementing all

bits

Data Memory

Location contents

(i) 0x020 0x2C

(j) 0x021 0xB2Complement all bits of k

In C:

k = ~k ;

In PIC18 assemblycomf 0x022, f ; k = ~k

(k) 0x022 0x8A

k = 0x8A = 1000 1010

After complement

result = 0111 0101= 0x75


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Bit set, Bit Clear, Bit Toggle instructionsCan set/clear/complement one bit of a data memory location by using the

AND/OR/XOR operations, but takes three instructions as previously

seen.

The bit clear (bcf), bit set (bsf), bit toggle (btg) instructions

clear/set/complement one bit of data memory using one instruction.

bcf floc, b [,a] 1 0 0 1 b b b a f f f f f f f f

B B B B B B B B B B B B B B B B1 1 1 1 1 1 0 0 0 0 0 0 0 0 0 05 4 3 2 1 0 9 8 7 6 5 4 3 2 1 0

bsf floc, b [,a] 1 0 0 0 b b b a f f f f f f f f

0 1 1 1 b b b a f f f f f f f fbtg floc, b [,a]

fffffff lower 7-bits of memory location

bbb three bit value that specifies affected bit


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Data MemoryBit clear/set/toggle

examples

Location contents

(i) 0x020 0x2C

(j) 0x021 0xB2Clear bit 7 of k, Set bit 2 of j,

complement bit 5 of i.

In C:

k = k & 0x7F;

j = j | 0x04;i = i ^ 0x20;

In PIC18 assembly

bcf 0x022, 7 ; k = bitclear(k,7)

bsf 0x021, 2 ; j = bitset(j,2)

btg 0x020, 5

(k) 0x022 0x8Abbbb bbbb7654 3210

k = 0x8A = 1000 1010

bcf k,7k = 0x0A = 0000 1010

j = 0xB2 = 1011 0010

bsf j,2j = 0xB6 = 1011 0110

i = 0x2C = 0010 1100btg i,5i = 0x0C = 0000 1100


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Conditional Execution using Bit Test

The bit test f, skip if clear (btfsc) and bit test f, skip if set

(btfss) instructions are used for conditional execution.

btfsc floc, b ; skips next instruction is bit b offloc is clear (0)

btfss floc, b ; skips next instruction is bit b offloc is set (1)

Bit test instructions used on status flags implements tests

such as equality (==), inequality (!=), greater than (>), less

than (


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(1) CBLOCK 0x0(2) loc,out ;byte variables(3) ENDC

(4) org 0

(5) goto main(6) org 0x0200(7) main(8) ;movlw 0 ;uncomment for loc=0(9) movlw 1 ;uncomment for loc=1(10) movwf loc ;initialize loc

(11) Ltop(12) btfsc loc,0 ; skip next if loc(0) is '0'(13) goto loc_lsb_is_1(14) ;LSb of loc = 0 if reach here(15) movlw 3 ; W 3

(16) movwf out ; out

(W)(17) movlw 2 ; W 2(18) movwf out ; out (W)(19) movlw 4 ; W 4(20) movwf out ; out (W)(21) loc_lsb_is_1

(22) movlw 8 ; W 8(23) movwf out ; out (W)(24) movlw 5 ; W 5(25) movwf out ; out (W)(26) movlw 6 ; W 6(27) movwf out ; out (W)(28) movlw 1 ; W 1(29) movwf out ; out (w)(30) goto Ltop ; loop forever(31) end

Skip goto loc_lsb_is_1

location if loc = 1.

Copyright Thomson/Delmar Learning 2005. All Rights Reserved.

Number Sequencing

Task using btfsc


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status Register

The STATUSregister is a special purpose register (like the wregister). The lower 5 bits of the status register are one bit flags

that are set/cleared as side effects of an instruction execution.

7 6 5 4 3 2 1 0

CDCZOVNuuu

u unimplemented

N negative flag (set if MSB of result = 1)

OV 2s complement overflow flag

Z zero flag (set if result = 0)DC decimal carry (carry/(~borrow) from bit 3 to bit 4)

C carry flag (carry/(~borrow) out of MSB

We will not discuss the DC flag, it is used in Binary CodedDecimal arithmetic.


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Carry, Zero Flags

Bit 0 of the status register is known as the carry (C) flag.

Bit 2 of the status register is known as the zero (Z) flag.

These flags are set as side-effects of particular instructions orcan be set/cleared explicitly using the bsf/bcfinstructions.

How do you know if an instruction affects C,Z flags?

Look at Table 20-2 in PIC18Fxx2 datasheeet.addwfaffects

all flags, movfonly Z,N flags.

Mnemonic Desc. Instr Mach. Code StatusCycles affected

ADDWF f,d,a |ADD WREG and F| 1|0010 01da ffff ffff|C,DC,Z,OV,NMOVF f,d,a |Move f | 1|0101 00da ffff ffff| Z,N


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Addition: Carry, Zero Flags

Zero flag is set if result is zero.

In addition, carry flag is set if there is a carry out of the MSB

(unsigned overflow, result is greater > 255)

0xF0

+0x20

--------0x10 Z=0,

C=1

0x01

+0xFF

--------

0x00 Z=1,

C=1

0x00

+0x00

--------

0x00 Z=1,

C=0

0x80

+0x7F--------

0xFF Z=0,

C=0


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Subtraction: Carry, Zero Flags

Zero flag is set if result is zero.

In subtraction, carry flag is cleared if there is a borrow from the

MSB (unsigned underflow, result is < 0, larger number subtractedfrom smaller number). Carry flag is set if no borrow occurs.

0x01-0xFF

--------

0x02 Z=0,

C=0

0x00-0x00

--------

0x00 Z=1,

C=1

0xF0- 0x20

--------

0xD0 Z=0,C=1

For a subtraction, the combination of Z=1, C=0 will not

occur.


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How do you remember setting of C flag

for Subtraction?Subtraction of A B is actually performed in hardware as

A + (~B) + 1

The value (~B) + 1 is called the twos complement of B(more on this later). The C flag is affected by the addition

of A + (~B) + 1

0xF0

- 0x20

--------

0xD0 Z=0,

C=1

0xF0+ 0xDF

+ 0x01

--------0xD0 Z=0,

C=1

0x20 = 0010 0000~0x20 = 1101 1111

= 0xDF

No borrow, C=1

Carry out of MSB,

so C=1


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CConditional Tests

logical AND&&

logical negation!

logical OR| |

less than, less than or equal=

equal, not-equal== , !=DescriptionOperator

If an operator used in a Cconditional test, such as an IF

statement or WHILE statement, returns nonzero, then the

condition test is TRUE.


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Logical Negation vs. Bitwise Complement!i ~iis not the same as

i = 0xA0 i = 0xA0

!(i) 0 ~(i) 0x5F

Logical operations: !, &&, || always treat their operands aseither being zero or non-zero, and the returned result is

always either 0 or 1.


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Examples ofCEquality, Inequality,

Logical, Bitwise Logical Testsunsigned char a,b,a_lt_b, a_eq_b, a_gt_b, a_ne_b;

a = 5; b = 10;

a_lt_b = (a < b); // a_lt_b result is 1a_eq_b = (a == b); // a_eq_b result is 0a_gt_b = (a > b); // a_gt_b result is 0a_ne_b = (a != b); // a_ne_b result is 1

unsigned char a_lor_b, a_bor_b, a_lneg_b, a_bcom_b;

(2) a = 0xF0; b = 0x0F;(3) a_land_b = (a && b); //logical and, result is 1

(4) a_band_b = (a & b); //bitwise and, result is 0(5) a_lor_b = (a || b); //logical or, result is 1(6) a_bor_b = (a | b); //bitwise or, result is 0xFF(7) a_lneg_b = (!b); //logical negation, result is 0(8) a_bcom_b = (~b); //bitwise negation, result is 0xF0



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if{} Statement Format in C

if_body and else_body can contain multiple

statements.

else_body is optional.


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Czero/non-zero tests

A Cconditional test is true if the result is non-zero; false ifthe result is zero.

The ! operator is a logical test that returns 1 if the operator is

equal to 0, returns 0 if the operator is non-zero.if (i) {// do this if i non-zero

j = i + j;}

if (!i) {// do this if i zero

j = i + j;}

Could also write:

if (i != 0) {// do this if i non-zero

j = i + j;}

if (i == 0) {// do this if i zero

j = i + j;}


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Cequality tests

== is the equality test in C; = is the assignment operator.

A common C code mistake is shown below (= vs == )

if (i = 5) {

j = i + j;} //wrong

if (i == 5) {

j = i + j;} // right

The test i == 5 returns a

1 only when i is 5. The

== is the equalityoperator.

Always executes

because i=5 returns 5,

so conditional test isalways non-zero, a true

value. The = is the

assignment operator.


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CBitwise logical vs. Logical AND

The & operator is a bitwise logical AND. The && operator

is a logical AND and treats its operands as either zero or non-

zero.

is read as:If ( (i is nonzero) AND

(j is nonzero) ) then do

this.

if (i && j) {/* do this */}

is read as:if (i & j) {/* do this */}

If ( (i bitwise AND j) is

nonzero) ) then do this.

i = 0xA0, j = 0x0B; i = 0xA0, j = 0x0B;

(i && j) 1 (i & j) 0x0


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CBitwise logical vs. Logical OR

The | operator is a bitwise logical OR. The || operator is

a logical OR and treats its operands as either zero or non-

zero.

is read as:If ( (i is nonzero) OR (j

is nonzero) ) { do...

if (i || j) {/* do this */}

is read as:if (i | j) {/* do this */}

If ( (i bitwise OR j) is

nonzero) ) { do....

i = 0xA0, j = 0x0B; i = 0xA0, j = 0x0B;

(i || j) 1 (i | j) 0xAB

Non-Zero Test labels for SFRs and


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Non Zero Test labels for SFRs andbit fields defined in

pic18f242.inc; usefor clarity!!!!

The movfinstruction just moves i back onto itself! Does nouseful work except to affect the Z flag.



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Conditional Execution using branches

A branch functions as a conditional goto based upon thesetting of a single flag

bc (branch if carry), bnc (branch if not carry)bov (branch on overflow), bnov (branch if no overflow)

bn (branch if negative), bnn (branch if not negative)

bz(branch if zero), bnz(branch if not zero)

bra (branch always)

Using branch instructions instead of btfsc/btfss generallyresults in fewer instructions, and improves code clarity.

Non-Zero Test


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Non Zero Test

The bz(branch if Zero, Z=1) replaces the btfsc/goto combination.

For a non-zero test if(!i){} replace bzwith bnz



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Equality Test (==)

Subtraction operation of i-j performed to check equality;

if i == j then subtraction yields 0, setting the Z flag. Does not

matter if i-j or j-i is performed.

Copyright Thomson/Delmar Learning 2005. All Rights

Reserved.

i h S i C


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switch Statement in C

A switch statement is a shorthand version of an if-else chain

where the same variable is compared for equality against

different values.Copyright Thomson/Delmar Learning 2005. All Rights

Reserved.

switch Statement in assembly language


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switch Statement in assembly language

A bra is a

branch always

unconditionalbranch. Copyright Thomson/Delmar Learning 2005. All Rights

Reserved.


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Unsigned greater-than Test (>)

If i > j, then j-i will result in a borrow (C=0). Subtraction

operation of j-i performed so test on C flag could be done.


Reserved.


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Unsigned greater-than Test (>) Variation

In this case, i-j is performed instead ofj-i. This requires checkingtwo flag conditions as the Z flag must be checked in case i is

equal to j. Obviously, the code is more complex because of the

need for two branches, so avoid doing the > comparison in thisway.Copyright Thomson/Delmar Learning 2005. All Rights

Reserved.


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Unsigned greater-than-or-equal Test (>=)

If (i >= j), then i j will produce no borrow if i > j or i == j.


Reserved.

Unsigned Comparison Summary


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Unsigned Comparison Summary

Z = 0

if i != k

i k OR i - kif ( i != k) {}

Z = 1

if i = = k

i k OR k - iif (i = = k) {}

C=1 (no borrow)

if i>= k

i kif (i >= k) {}

C = 0 (borrow)

if i > k

k iif (i > k) {}

If true, thenOperationComparison

If you do i-k for the i>k comparison, then will have to test both

C and Z flags. If you do k-i for the i>=k comparison, then will

have to test both C and Z flags.

PIC18 Comparison Instructions


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PIC18 Comparison Instructions

The PIC18 has three instructions that directly implement ==,> (unsigned), and < (unsigned).

cpfseq floc ; iffloc == w, skip next instruction

cpfsgt floc ; iffloc > w, skip next instruction

cpfslt floc ; iffloc < w, skip next instruction

You can use these to implement 8-bit, unsigned comparisons infewer instructions than using the subtract/flag test approach.

HOWEVER, the subtract/flag test is a more general approach,

and is useful for any comparison type ( 8-bit unsigned/signed,

16-bit signed/unsigned, etc). The subtract/flag test is emphasized

in this class because it can be generally applied; the comparison

instructions only work for 8-bit unsigned comparisons.

Comparison example (==) using cpfseq


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Comparison example (==) using cpfsequnsigned char i,j;

if (i == j) {j = i + j;

}

using cmpfseqsubtract/Z flag test

movf j,w ; w jsubwf i,w ; w i jbnz end_if ;skip if Z=0

movf i,w ; w iaddwf j,f ; j j + wend_if..do stuff..

movf i,w ; w icpfseq j ; j == i?bra end_if ; i != j

addwf j,f ; j

j + iend_if..do stuff..

Ccode

did j==i so that w already had

i in it to reduce instr. count

The cmpfseq approach takes fewer instructions. Use this approach

if you want to, but you will still have to understand the

subtract/flag test approach for other comparison types later.

Comparison example (>) using cpfsgt


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Comparison example (>) using cpfsgt

using cpfsgtsubtract/C flag test

movf j,w ; w jcpfsgt i ; i > j?bra end_if ; i j) {j = i + j;

}

movf i,w ;wisubwf j,w ; wjibc end_if ; skip if C=1

movf i,w ; wiaddwf j,f ; jj + w

end_if

..do stuff..

No advantage the subtract/flag test is more general and

you will need to understand it as it is used for other

comparison types later.

Unsigned Literal Comparison


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Unsigned Literal Comparison

Careful! The sublw instruction performs literal w,

not w literal.

This is useful for comparing w to a literal.


Reserved.

while loop


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p

Observe that at the end of the loop, there is a jump back to

loop_top after body is performed. The body of a while

loop may not execute if loop test is initially false.


Reserved.

do-while loop


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In do-while loop, body is always executed at least once.


Reserved.

Aside: for loops in C


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Aside: forloops in C

A forloop is just another way to write a while loop.

Typically used to implement a counting loop (a loop that is

executed a fixed number of times).

unsigned char i,j;

i = 0;

while (i != 10) {k = k + j;i++;

}/* ..do stuff..*/

unsigned char i,j;

for (i = 0; i!= 10; i++){k = k + j;

}/* do stuff */

These statements executed 10 times. Both code blocks

are equivalent.

executed once,

before test.

executed eachloop iteration

after body

loop test

Decrement/Increment skip if 0 !0


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Decrement/Increment, skip if 0, !0

For simple counting loops, where goal is to execute a block ofstatements a fixed number of times, the decrement/increment,

skip if 0 instructions can be useful.

decfsz floc ; decrement floc, skips next instruction if result == 0

dcfsnz floc ; decrement floc, skips next instruction if result != 0

incfsz floc ; increment floc, skips next instruction if result == 0

infsnz floc ; increment floc, skips next instruction if result != 0

Can use these for counting loops; replaces multiple instructionswith single instruction. The reason to use these instructions

would be to save code space, and decrease loop execution time.

Counting Loop Example


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Counting Loop Example

Can be used for an end-of-loop action and test in some cases.

Usage of incfsz/icfsnz/decfsz/dcfsnz is optional as other

instruction sequences can accomplish the same thing. In this

case the decfsz/bra can be replaced by decf/bnz

combination that is just as efficient.Copyright Thomson/Delmar Learning 2005. All Rights

Reserved.

C i d Shift L ft Shift Ri ht


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Cunsigned Shift Left, Shift Right

unsigned Shift right i >> 1

all bits shift to right by one, 0 into MSB.

b7 b6 b5 b4 b3 b2 b1 b0 original value

i >> 1 (right shift by one)0 b7 b6 b5 b4 b3 b2 b1

unsigned Shift left i


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PIC18 Rotate InstructionsPIC18 has rotate right through carry, rotate right, rotate left through

carry, rotate leftinstructions

rrcf floc, d ; d shifted to right by 1, MSB gets C flag,

LSB goes into C flag

b7 b6 b5 b4 b3 b2 b1 b0Cflagrotate

right thru

carry

rlcf floc, d ; d shifted to right by 1, LSB gets C flag,

MSB goes into C flag

b7 b6 b5 b4 b3 b2 b1 b0Cflagrotateleft

thru

carry

PIC18 Rotate Instructions (cont)


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( )

rrncf floc, d ; d rotated to right by 1

rotate

right, no

carryb7 b6 b5 b4 b3 b2 b1 b0

rlncf floc, d ; d rotated to left by 1

rotate

left, nocarry

b7 b6 b5 b4 b3 b2 b1 b0

C Shift operations using Rotates thru Carry


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C Shift operations using Rotates thru Carry

For multiple shift, repeat single shift. Must clearCarry flag as

status is unknown usually.


Reserved.

Wh Shift?


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Why Shift?

Shift right by 1 is divide by 2 ( i >> 1 == i / 2 )

Shift left by 1 is multiply by 2 ( i


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y

The PIC18 instruction set has gotten cluttered with instructions asnew generations of PICmicro microcontrollers have been produced.

New instructions were added, and old instructions were kept in an

effort to keep assembly source compatibility (so that compilerscould be ported more easily).

There are many ways for accomplishing the same thing using

different instructions sequences.

Which method do you use?

The method that you understand......(and have not MEMORIZED).

I will never take off points for inefficient code.

I will always take off points for incorrect code close does not

count.

What do you need to know?


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What do you need to know?

Logical operations (and,or,xor, complement)

Clearing/Setting/Complementing groups of bits

Bit set/clear/test instructions ==, !=, >, =, >), Shift Right (

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Chap4 Logctrl Ops

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